To provide improved protection of the non-metal part against thermal shock, the present assignees' U.S. Pat. No. 3,964,536 discloses one or several closely applied metal parts on at least part of the head, the metal part advantageously being in the form of a sleeve made of a metal which is a good heat conductor (and/or a ring) crimped on the head.
In some circumstances, the invention of the above-mentioned patent gives rise to difficulties. Indeed, with an aluminium or aluminium alloy sleeve crimped on the head of a glass electrically insulating part, the combined effect of temperature and pressure during injection of the metal for forming the moulded cap can cause a degree of adherence of the sleeve- constituting metal sheet on the tempered glass surface, especially in the zone directly subjected to the molten metal. Such adherence is undesirable, since on cooling, small cracks can appear in the surface of the glass down to a depth of about 5 to 50 microns.
To solve this problem it was then contrived to use lubricants or mould-stripping products such as molybdenum bisulphide aerosols, for example. However, this is not entirely satisfactory, since fairly frequently the surface of the glass still adheres to some degree to the metal sheet in the hottest zone; further, under the effect of heat, the mould-stripping agents or the dispersion fluids can evolve gas which forms bubbles between the metal sheet and the glass.
Preferred embodiments of the present invention improve the method of the above-mentioned patent and considerably reduce the danger of adherence of the metal on the tempered glass surface, particularly in the zone in which the effects of temperature and pressure combine when the molten metal is injected.